The only BYOD device you can use on the Sprint Unlimited plan for LTE is a Sierra Wireless Tri-Mode Hotspot ... pricey. There are no Sprint 4G/LTE aircards on the market yet.
Then you have the option with the Millenicom Hotspot plan on Verizon, 4G/LTE with a 20GB cap. -- ... need help? »evdo-tips.com/
I just spoke with Millenicom and they said that although Verizon and Sprint are available to me, it looks like they haven't allowed Millenicom access to their towers in my area? I didn't realized it worked that way.